fix compile in gcc:release and support string_view to_json

[bbbgan]

No description provided.

[codecov-commenter]

Codecov Report

Patch coverage: 84.61% and project coverage change: +0.04 🎉

Comparison is base (a1ff066) 43.74% compared to head (96331e0) 43.78%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   43.74%   43.78%   +0.04%     
==========================================
  Files          36       36              
  Lines        6502     6512      +10     
==========================================
+ Hits         2844     2851       +7     
- Misses       3658     3661       +3     

Impacted Files	Coverage Δ
example/json_example.cpp	0.00% <0.00%> (ø)
iguana/json_writer.hpp	76.47% <100.00%> (+1.47%)	⬆️
iguana/xml_reader.hpp	100.00% <100.00%> (ø)
test/unit_test.cpp	99.56% <100.00%> (+<0.01%)	⬆️

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